The poll, conducted by Democratic pollster David Binder for the group Focus on Rural America, shows Warren leading the field with 20 percent of the vote, a statistical tie with Harris at 18 percent and Biden at 17 percent.

The poll shows Warren and Harris both doubling their support following strong performances in last week’s debate, while Biden and Sanders have slipped. In the group’s March poll, Biden led the field with 25 percent support, followed by Sanders at 17 percent.

Klobuchar, Booker and former Rep. Beto O’Rourke (D-Texas) all saw their poll numbers slip. O’Rourke’s support slipped from 6 percent in the March poll to just 1 percent in the July survey, conducted entirely after the debates.

The debates certainly helped Harris and Warren: Voters who watched some or all of the two-night coverage said they were most likely to back one of the two senators, both of whom stood out onstage against their rivals.

Harris has been steadily rising since she announced she would run in January.

Today, 82 percent of Iowa Democrats surveyed say they have a positive opinion about the California senator, up from 54 percent in a December poll and 73 percent in March. Warren is the most popular Democrat in the field, with 88 percent of respondents saying they have a positive opinion about the Massachusetts Democrat.

Buttigieg’s image has also improved markedly, growing from a 44 percent positive image to an 80 percent positive image from March to July.

On the other hand, Biden and Sanders have both seen opinions about them slide.

About three-quarters of Iowa Democrats polled, 74 percent, see Biden positively, down from 90 percent in December and 88 percent in March; Sanders’s positive image slipped from 83 percent in March to 73 percent in July.